The present invention generally relates to electrical measuring instruments, and more particularly to an improved digital volt-ohmmeter capable of measuring milliohm resistances with precision and high accuracy.
The heart of any digital volt-ohmmeter is an analog-to-digital converter. Typically, the converter has an analog section and a digital section. The analog section may comprise a high gain differential amplifier connected to receive an input voltage across its input terminals and to provide an output voltage to one input of an analog comparator. The other input of the analog comparator in connected to a source of reference voltage, and when the voltages applied to the two inputs of the analog comparator are equal, an output signal indicative of this condition is generated. The digital section of the analog-to-digital converter may comprise a source of clock pulses connected through a gate to a digital counter. The output signal from the analog comparator is used to develop a gating signal so that the accumulated count in the digital counter is proportional to the voltage connected across the inputs of the high-gain differential amplifier. At the end of the measurement, the count accumulated in the digital counter may be either read out or displayed in a well known manner.
Problems arise in the measurement of very small voltage or resistance values. For example, it is often desirable to reliably and accurately measure the resistance of connections to integrated circuits. Such resistances are in the range of millohms. In order to measure such very small resistance values, it is necessary to significantly increase the gain of the input differential amplifier. Very high gains on the order of 1000 are often necessary; however, such high gains are usually accompanied by amplifier drift which adversely affects the accuracy of the measurement made. Moreover, because of the very high gain amplifier required for the measurement of such small resistances, common mode voltages to the amplifier cannot be tolerated.